blackest Loc: Ireland

Found a youtube video
add a new layer set it to 50% Gray and blend mode to difference
and a new layer threshold and move the slider all the way to the left move it right until some black appears
use the color sampler tool to select a black pixel.
turn off these 2 layers
and add a levels or curves adjustment layer
use alt and black dropper for blacks to set the black point
alt and white dropper for whites to set the white point
select the pixel you selected with the color sampler tool to set the gray point

Seems to work.

BradJP Loc: Omaha, NE

Whibal card works great.

GENorkus Loc: Washington Twp, Michigan

Simple enough. Before needing it go to a paint or hardware store that sells paint. Grab a free paint chip card of whatever color you feel good with and place it in your wallet or camera bag.

BobHartung Loc: Bettendorf, IA

I frequently white balance on anything of a mid green color.

dandev Loc: Enumclaw, WA

I have a small collapsible gray "card" - similar to this one from B&H. It's about 4 inches across when folded and stuffs in a corner of my photo bag. As some of the others have posted - it's a place to start your WB calibration.

https://www.bhphotovideo.com/c/product/981245-REG/impact_qbp_g_12_quickbalance_panel_18_percent_gray_12.html/?ap=y&gclid=CjwKCAjwndvlBRANEiwABrR32MAtNXvbhffcualLjYdCOPjMuzwxeAUJtG1BwJz7ZFdrwG2EMQu7EBoCLG8QAvD_BwE&lsft=BI%3A514&smp=Y

DirtFarmer Loc: Escaped from the NYC area, back to MA

The trick is finding that pixel again.

amfoto1 Loc: San Jose, Calif. USA

A lot of responses seem to address using a gray card to set exposure... which is one use for it.

The question was about setting Custom White Balance, which is different... and also can be done with a gray card, so long as it's TRULY neutral gray.

To set a Custom WB it doesn't have to be gray... can be pure white or pure black, too.

However, even those are hard to find. The slightest tint will skew your results, trying to set WB after the fact like you're doing.

I suggest you get a small or folding Custom WB target to use when you are taking the shot, instead.

I use a Lasotite EZ Balance target (https://www.bhphotovideo.com/c/product/330874-REG/Lastolite_LL_LR1250_EZ_Balance_Grey.html. It's fabric with a spring steel frame that folds to about 1/3 it's size for storage. It has a pure white side and an 18% gray side. Either side can be used to set a Custom WB. I usually use the gray side because the same readings and WB sample image image can be used to help establish an accurate exposure. If you don't want to set a Custom WB when shooting, you also can simply incorporate the target in a shot that can later be used in LR or PS (or whatever) to fine tune color.

I also carry Warm Cards, by Vortex Media. These are smaller, rigid cards that have been laminated for safe keeping. Included is a standard gray card (white on the opposite side). But more often I use the warming or cooling cards to set slightly biased Custom WB. There's also a card included to set Custom WB specifically for fluorescent lighting. They come in two sizes... 4.5x6" cards or 6x9" cards. This link is a set including both sizes: https://www.bhphotovideo.com/c/product/809216-REG/Vortex_Media_WC3_WC3_WarmCards_3_0_White.html. More info about them and some examples where they've been used can be seen here: https://www.vortexmediastore.com/pages/warmcards-white-balance-system.

scsdesphotography Loc: Southeastern Michigan

To set a custom WB, I use Dot-Line WB lens caps. They're convenient, reliable and relatively inexpensive. The caps are available in various sizes and I have one on each of my lenses. Btw, B&H just restocked these handy caps, for the price I suggest you give one a try. In addition to the other suggestions green grass also meters as grey.

CatMarley Loc: North Carolina

A hankerchief, a tissue, a white undershirt, a sock? You have none of the above while hiking?

no12mo

That 4 x 6 grey card works for me. Just cut them so that they are 3 x 4 and go with that. The skin reference not so much.

The grey card also allowed you to adjust manual exposure back in the day. Today's instant gratification technology is much more accurate

marty wild Loc: England

Cut a small bit of grey card double sided tape In an lens filter

blackest Loc: Ireland

It can be quite difficult to find a genuine gray, Gray with a touch of blue is remarkably common.

marty wild Loc: England

From eBay I purchased a grey card which was the same size as a credit card on a neck strap. It works for me and it would cut with craft scissors,

burkphoto Loc: High Point, NC

This thread is about achieving WHITE BALANCE. Yes, blue jeans, green grass, etc. can be used for a “wet finger in the wind” exposure, but they’ll surely result in UGLY white balance.

sbohne

I'm sorry, this is not 100% accurate.

Regardless of your skin pigmentation, there is NO pigmentation in your palms...or the soles of your feet. That is why the read of the palm method works regardless of your skin color.